The best way setup a hidden gallery will be to..
First create your gallery per normal and upload some images. (Lessons 2 and 3 in our Kick Start Guide will cover this)
Then access the new gallery on your live site and copy down the gallery's URL.
- You can now head to the Gallery Edit page under your Library / Galleries section. Click the Privacy & Security bar and set the gallery's Portfolio Visibility toOff.
This way the gallery will still be accessible directly via the URL, however it won't be displayed on your portfolio page. So you can send the URL to specific customers and only they will be able to find the gallery.